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Driver for an oled passive-matrix displayDriver for an oled passive-matrix display description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070171155, Driver for an oled passive-matrix display. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to displays, and more particularly to a driver for an Organic Light-Emitting Diode (OLED) passive-matrix display. [0003] 2. Description of the Related Art [0004] Liquid crystal displays (LCDs) are the most common type of flat-panel display used today. One drawback, however, to LCDs is that they require a separate light source, typically a fluorescent backlight, to illuminate the panel. In fact, the LCD's brightness depends solely on its backlight and it is this backlight that limits the life of the LCD. [0005] Because of these drawbacks, OLED displays are gaining in popularity. OLED displays are self-luminous and, therefore, do not require a separate backlight. Passive-matrix OLED displays have a simple structure and are well suited for low-cost and low-information-content applications, such as alphanumeric displays. Active-matrix OLEDs have an integrated electronic backplane that enables high-resolution, high-information-content applications, including videos and graphics. In any event, the OLED displays are very thin, compact displays with wide viewing angles (up to 180 degrees), fast response, high resolution, and good display qualities. [0006] The basic OLED cell includes a stack of thin organic layers sandwiched between an anode and a metallic cathode. The organic layers generally include a hole-injection layer, a hole-transport layer, an emissive layer, and an electron-transport layer. The emissive layer is primarily responsible for the light generation or electroluminescence. Specifically, when an appropriate voltage is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light. The structure of the organic layers, of the anode and cathode is designed to maximize the recombination process in the emissive layer, thereby maximizing the light output from the OLED display. [0007] The light output or brightness of an OLED display is directly proportional to current flow. Additionally, the impedance of the OLEDs drops exponentially with an increasing forward voltage (VF). Thus, as impedance drops, light output increases rapidly and there is virtually no delay between the generation of current flow and the generation of light output. [0008] One problem with OLED displays is the variation of the current-voltage (I-V) characteristics over time, which causes degradation of the luminance efficiency and pixel-to-pixel luminance uniformity. Several factors contribute to this variation in the I-V characteristics including operating temperature, external light (e.g., sunlight), pixel position on the display, etc. The driving method also affects the I-V characteristics. For example, in an OLED passive-matrix display, one method used is called multiplexing line address (MLA), wherein the average current needed to bias the OLED is multiplied by the duty cycle of the row to compute an equivalent multiplexing current, which may be 50 to 200 times the average bias current (1 .mu.A to 1 mA from dim to bright) depending on the number of rows and the efficiency of material. Such high currents cause excess voltage drops on the OLEDs that results in wasted power consumption. [0009] International application WO 03/107313A2 to Cambridge Display Technology Limited discloses a technique to reduce power consumption in an active-matrix display by using current and voltage sensors and by controlling an adjustable power supply that adjusts the voltage in response to the sensed voltage. However, this application only discloses indirectly measuring voltage and current used by the display pixels, which is less desirable. Additionally, there is no well-defined technique disclosed for efficient power-up of the OLED display. That is, when the display is first powered on, the pixels are off and the required voltage needed by the OLED display is not well defined. [0010] Thus, there is a need for a display that can efficiently bring the OLEDs through a power-up mode and allow for adjustment of the power levels supplied to the OLEDs after the power-up mode has been completed. BRIEF SUMMARY OF THE INVENTION [0011] In order to overcome the deficiencies of the prior art, an OLED passive-matrix display is disclosed that allows for an efficient power-up mode of operation, as well as the ability to adjust power (e.g., voltage and/or current) supplied to the OLEDs based on need during normal, steady-state conditions. [0012] In one embodiment, the OLED passive-matrix display includes a monitor circuit that monitors the real-time voltage levels used by the OLEDs and a voltage adjusting circuit that changes the supply voltage in response to signals received from the monitor circuit. During a power-up mode, the voltage adjusting circuit uses a fixed reference voltage as a basis for generating supply voltage when the power needed by the OLEDs is not well defined. But after a predetermined period of time or in response to an external signal, the voltage adjusting circuit switches from reading the fixed reference voltage to reading a variable voltage level supplied from the monitor circuit. This variable voltage is based on voltage readings of the OLEDs, such as reading the voltage drops directly across the OLEDs. In response to this variable voltage level, the voltage adjusting circuit modifies the voltage supplied to the OLEDs. In this way, there is no wasted power dissipation and the circuit has real-time tracking of all the OLEDs. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS [0013] One example embodiment of the present invention is now described, which proceeds with reference to the following drawings: [0014] FIG. 1 is a circuit diagram of a display portion of an OLED passive-matrix display. [0015] FIG. 2 is a high-level block diagram of an OLED passive-matrix display according to one example embodiment of the invention. [0016] FIG. 3 is a detailed circuit diagram showing further features of the block diagram of FIG. 2. [0017] FIG. 4 is a flowchart of a method for operating the OLED passive-matrix display. DETAILED DESCRIPTION OF THE INVENTION [0018] FIG. 1 shows a display portion 10 of an OLED passive-matrix display. A matrix 12 of OLEDs 13 includes parallel rows 14 of conductors positioned orthogonally to parallel columns 16 of conductors. Each row 14 includes OLED Dx1 to Dxm (where x is the row number and m is the number of columns), and each column 16 includes OLEDs D1x to Dnx (where n is the number of rows and x is the column number). Each column is biased with a current generator 18 (1 to m) coupled at its upstream end to a voltage source VH and at its downstream end to one of the column switches SC1-SCm. Each row 14 includes one of the row switches SR1-SRn with its upstream end coupled to the OLEDs, and its downstream end coupled to a cathode 21. Column switches SC1-SCm and row switches SR1-SRn are independently switchable so that each OLED can be selected individually irrespective of the other OLEDs. To measure voltage directly across the OLEDs, voltage taps 20 are coupled to the columns as indicated at VFD1-VFDm. These voltage taps 20 may be coupled upstream or downstream of the switches SC1-SCm, and the taps 20 can be used to read the OLED voltages externally. [0019] The voltage source VH must have a high enough voltage to account for the OLED "ON" voltage, the voltage drop on the rows 14 and columns 16, the voltage saturation of the current generators 18, and the voltage drop on the switches (SC1-SCm and SR1-SRn). A driver circuit, not shown in FIG. 1 but described below, is used to generate the power supplied from the voltage source VH. [0020] In operation, the display portion 10 performs a scan operation wherein one row is activated at a time through successive activation of switches SR1-SRn. However, the frequency is such that the activation and deactivation of the OLEDs is not detectable to the human eye. Because only one of the row switches SR1-SRn is activated at a time, the voltage taps 20 are used to read a voltage drop directly across one OLED in a column at a time. Such a direct measurement is a very accurate way of determining the voltage used by each OLED in the display. 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